Potential hybrid feedstock for biodiesel production in the tropics

Abstract

Recently, mixture of different oils at various proportions have been used as feedstock for biodiesel production. The primary aim is to improve fuel properties which are strongly influenced by the fatty acid composition of the individual oil that makes up the feedstock mix. The tropics are renowned for abundant oil-bearing crops of which palm kernel oil (PKO) from palm seed and groundnut oil (GNO) are prominent. This present paper investigated biodiesel production from hybrid oil (HO) of PKO (medium carbon chain and highly saturated oil) and GNO (long carbon chain and highly unsaturated oil) at 50/50 (v/v) blending. The principal fatty acids (FAs) in the HO are oleic (35.62%) and lauric acids (24.23%) with 47.80% of saturated FA and 52.26% of unsaturated FA contents. The chemical conversion of the oil to methyl ester (ME) gave 86.56% yield. Fuel properties of hybrid oil methyl ester (the HOME) were determined in accordance with standard test methods and were found to comply with both ASTM D6751 and EN 14214 standards. The oxidative stability, cetane number and kinematic viscosity (KV) of HOME were observed to be improved when compared with those of GNO methyl ester from single parent oil, which could be accredited to the improved FA composition of the HO. The KV (3.69 mm²/s) of HOME obtained in this paper was remarkably low compared with those reported in literature for most biodiesels. This value suggests better flow, atomization, spray and combustion of this fuel. Conclusively, the binary blend of oils can be a viable option to improve the fuel properties of biodiesel feedstock coupled with reduced cost.

Keywords

groundnut oil / palm kernel oil / methyl ester / fuel properties / tropics / fatty acid composition

Introduction

Emission-related issues and persistent fuel crises associated with petroleum and its derivative fuels have reawakened attention for alternative and sustainable fuels of biological source, mainly referred to as biofuel. Biodiesel is a type of biofuel widely accepted as a substitute fuel to diesel fuel. Energy security and independence is crucial to economic development of nations and biodiesel has been accepted in this regard, especially by European countries which have recorded giant stride in this direction by the increase in quantity of biodiesel production.

Biodiesel is considered a regional fuel because oil-bearing crops from which biodiesel is mostly produced are geographically influenced. Each region of the world has oil crops available in the region that can be explored for biodiesel production. A wide variety of oils can be used for biodiesel production. These include vegetable oils (edible and non-edible oils),animal oils and food processing waste (waste cooking oils, animal oils and fat, tallow, lard, yellow grease, chicken fat) [ 1– 4], insect oils [ 5, 6], and other oil-based wastes and residues [ 3, 7– 9]. The cost of biodiesel production is between 60–90% of the cost of feedstock. Presently, over 95% of global biodiesel production is obtained mainly from edible VOs [ 3] despite the price increase and competition with food and non-food uses. Low-cost oils are expected to help reduce the cost of feedstocks utilized for biodiesel production.

Several studies were conducted on biodiesel production using single oil [ 1– 3, 10, 11]. Conversely, a number of researches were carried out on the mixture of different feedstocks for biodiesel production [ 12– 16]. The personal observation of previous studies showed that most of these studies were conducted for binary mixture with few studies on ternary [ 17, 18] and quaternary mixtures [ 16, 19]. Recently, Almeida et al. [ 18] studied biodiesel production from mixtures of waste fish oil, palm oil and waste frying oil coupled with the optimization of fuel properties. It was reported that waste fish oil (42.1% (wt.)) and waste fryingoil (57.9% (wt.)) mix is advantageous, if the aim is to improve IP (20%) and COM (80%). Meneghetti et al. [ 12] studied the viscosity of biodiesels produced from castor oil mixed with soybean and cottonseed oils, respectively. Taravus et al. [ 14] investigated the fuel properties of the mixture of sunflower oil and beef tallow biodiesel. They observed that biodiesel produced from 40% of beef tallow and 60% of sunflower oil blend gave satisfactory and improved fuel properties. Karanja and mahua (50:50 v/v) oil was studied as a hybrid feedstock for efficient biodiesel production in India [ 20]. Qiu et al. [ 21] studied the mixtures of soybean oil and rapeseed oil and found improved fuel properties close to that of commercial diesel fuel. In addition, Chen et al. [ 22] investigated the blending of palm kernel oil (PKO) and coconut oil biodiesels with tung oil biodiesel and discovered improvement in fuel properties of tung oil biodiesel.

Report has it that PKO and groundnut oil (GNO) are one of the underutilized and highly neglected oils available for biodiesel production in Nigeria [ 23] which is a tropical region. Few studies utilized PKO and GNO as single-material feedstock for biodiesel production [ 24– 26]. PKO is an edible oil obtained from fruits of tropical oil-bearing palm trees. It is primarily composed of saturated medium-chain FAs (C6-C14; mainly lauric acid (C12:0)) [ 1]. The world producers of PKO are Indonesia, Malaysia, Nigeria, Thailand, and Papua New Guinea, with world production estimated around 5.50 million metric tons in 2008/2009 [ 22]. Saturated FAMEs (such as palm oil biodiesel) have a relatively high value of kinematic viscosity with a fairly low iodine value as well as a good oxidative stability and cetane number, but poor cold flow properties [ 27]. Groundnut or peanut (Arachis hypogea L.) is an important crop grown mainly in the tropics [ 28]. Most groundnuts grown in the world are used for oil production, peanut butter, confections, and snack products [ 25]. It is a non-drying oil with unsaturated long chain FAs (C16-C24; mainly oleic acid (C18:1)) [ 28]. Oils with unsaturated FAMEs (such as soybean oil biodiesel) have a relatively high iodine value and a poor oxidative stability [ 27].

Subject to the aforementioned, methyl ester with complementary characteristics is expected from the blend of saturated medium-chain (PKO) and unsaturated long-chain (GNO) FA oils. In this present paper, the HO obtained from the mixture of PKO and GNO was investigated as potential feedstock for biodiesel production in the tropics from a Nigeriain perspective. The objective of this paper was to improve the deficiencies in the fuel properties of both PKO and GNO biodiesels, though information concerning the fuel properties of the biodiesels was scarce in literature.

Materials and methods

Materials

Five liters of PKO were bought from a palm kernel processing factory in Iyanu-Ipaja, Lagos State, Nigeria while the same liters of GNO were purchased from a local market in Ifo, Ogun State, Nigeria.

Reagents

Reagents such as methanol 99.5% (Fischer Scientific), sodium hydroxide pellet 95% (Aldrich), sodium sulphate (Aldrich), hydrochloric acid 37% (Aldrich), heptane 99.5% (Fluka), methyl heptadecanoate 99.7% (Fluka), and triacetin 99% (ALFA AESAR) were purchased from AKEST chemical store in Jankara, Lagos State, Nigeria. All the chemicals were of analytical reagent grade.

Hybrid oil preparation and pretreatment

Two parent oils (GNO and PKO (50/50 (v/v)) were mixed together after weighing 500 ml of each in a two-liter conical flask using measuring cylinders. The mixed oil was then filtered using filter paper (doubled) to remove the solid and insoluble particles present in it. Moisture was removed by heating the mixed oil in oven up to 105 °C for 2 h and was stored at 10 °C until used. Pretreatment of the oil mixture became necessary due to the crude nature of the oils and in order to eliminate the presence of water which would reduce ester yield during chemical conversion.

Physicochemical properties of hybrid oil

The oil quality of the oil mixture (HO) was expressed in terms of its physicochemical properties which were obtained using standard test methods. The iodine value, acid value, density, kinematic viscosity and free fatty acid (FFA) were determined using AOAC test methods [ 11]. These were conducted in triplicate and the average values reported.

Transesterification reaction

A single-step transesterification process was conducted because the HO has an acid value (1.67 mg KOH/g) of less than 2 mg KOH/g recommended for such reaction to take place [ 11]. 250 g of the HO was measured and poured into the reactor. The reaction was conducted under classic transesterification conditions: a molar ratio of methanol to the HO of 6:1, a rotational speed of 600 r/min, a reaction temperature of 60°C, a reaction time of 1 h, and a catalyst (NaOH) dosage based on an oil weight of 1% w/w. Appropriate quantity of methanol and NaOH was measured (using weighing balance) and poured into a beaker with the content stirred vigorously to completely dissolved NaOH in methanol. The mixture (sodium methoxide) obtained was poured into the reactor (containing the HO) earlier preheated and maintained at 60°C, which signaled the commencement of the reaction. Besides, the stirring (using magnetic stirrer) of the content of the reactor was started at a stirring rate of 600 r/min immediately after the reaction began. The reaction was allowed for 1 h, after which the heating and stirring was stopped and the reaction product left to cool at ambient temperature. The mixture was left overnight to settle, a process that resulted in phase separation. It was observed that the resulting mixture had settled into two distinct phases of glycerol (bottom) and pale yellow ester (above).The excess methanol remaining in the crude ester mixture was removed by evaporation (using rotary vacuum evaporator at 90°C for 1 h under reduced pressure). Subsequently, the ester phase was washed three times with warm distilled water (three times the volume of the ester phase) to remove any residual methanol, NaOH or glycerol that might have been present. Finally, trace amounts of water were removed by adding anhydrous sodium sulphate, followed by filtration and stored in cool, dry place before carrying out further analyses on it. The procedure was replicated three times and the average biodiesel weight as well as glycerol weight obtained was measured.

Biodiesel characterization

Selected fuel properties of the hybrid oil methyl ester (HOME) were measured according to standard biodiesel test methods. The acid value (ASTM D664), iodine value (EN 14111), kinematic viscosity (ASTM D445), cloud point (ASTM D2500), water content (ASTM D95), density (ASTM D5002), flash point (ASTM D93), pour point (ASTM D97), oxidation stability (EN 14112), and higher heating value (ASTM D4868) were determined experimentally, whereas the cetane number was evaluated empirically. In addition, the free and total glycerol, and FAME composition of HOME were analyzed according to ASTM D6584 and EN 14103, respectively, using an Agilent 6890N gas chromatograph (Santa Clara, CA, USA) with a flame ionization detector (GC-FID). Standard procedures earlier reported were strictly followed [ 11].

Results and discussion

Physicochemical properties of hybrid oil

Table 1 provides the physicochemical properties of the HO and those previously reported for PKO and GNO for comparison. It can be observed from Table 1 that the density, acid value (AV), iodine value (IV) and kinematic viscosity (KV) of the HO are 911.01 kg/m³, 1.41 mg KOH/g, 53.73 g I₂/100g and 31.53 mm²/s, respectively. An AV of 1.41 mg KOH/g corresponding to 0.71% FFA necessitates the direct transesterification process. The KV (31.53 mm²/s) and density (911.01 kg/m³) obtained in this paper were compared with traditional oils as previously reported [ 29] and were found to be in good agreement. It appears that the AV, density, IV and KV of the HO are slightly lower and slightly higher than that GNO and PKO, respectively, as provided in Table 1. This may be accredited to the significant differences in the FA compositions of the starting oils (PKO and GNO) as given in Table 1. Table 2 lists fatty acid compositions of PKO, GNO and HO.

Fatty acid profile of hybrid oil

The FA profiles of the hybrid, palm kernel and groundnut oils are given in Table 1. As clearly observed in Table 1, the HO primarily consists of lauric (C12:0; 24.23%), oleic (C18:1; 35.62%) and linoleic (C18:2; 15.23%) acids, which are contributed by GNO and PKO, respectively. From Table 1, it can be observed that the single oil of PKO contained 80.4% of saturated and 19.6% of unsaturated FAs while GNO contained 15.5% of saturated and 84.5% of unsaturated FAs. However, the HO has a saturated FA of 47.80% and an unsaturated FA of 52.26%, which is evident of the mixture of PKO (highly saturated oil with medium carbon chain) and GNO (highly unsaturated oil with long carbon chain). This result is also evidently proved by the average molecular weights of 712.07 g/mol for PKO (medium carbon chain) and 890.29 g/mol for GNO (long carbon chain) compared to that of HO (801.35 g/mol).

Yield of hybrid oil methyl ester

The weights of methyl esters prepared, the weights of reactants and the weights of glycerol of three runs at the experimental conditions are given in Table 3. HOME afforded an average yield of 86.56% (216.40 g) from 250 g of the HO, 60 g of methanol and 2.5 g of NaOH at the classic reaction conditions (Table 3). It is worth noting that this yield seems satisfactory having been obtained from oils in crude form and that this yield value is obtained from classic reaction conditions and not from optimized reaction conditions. The negative effect of impurities present in oil on ester yield were stressed [ 27] and that with the same reaction conditions, 67%–86% of ester yield can be obtained using crude VOs, compared with 93%–98% when using refined oils [ 32]. To this effect, the HOME yield achieved in this present paper is well within the range of values reported in previous studies [ 33– 35]. For industrial application of HOME, the reaction conditions must be optimized and the oils refined.

Characterization of hybrid oil methyl esters

The FA composition of biodiesel feedstock is known to significantly influence fuel properties of biodiesel [ 36]. The distinct FA composition of the HO (blend of medium and long carbon chains) obtained from PKO and GNO blend is expected to produce fuel properties different from those of PKO and GNO biodiesels.

Kinematic viscosity

Viscosity is the most important biodiesel property because it affects the operation of fuel injection equipment, especially at low temperatures when an increase in viscosity affects the fluidity of the fuel. The KV of HOME was measured to be 3.69 mm²/s which is remarkably lower than the values of≥4.0 mm²/s reported for biodiesels (Table 4). Similarly, low values of KV as that of this present paper, were reported in literature for biodiesels synthesized from cuphea oil (2.38 mm²/s), PKO (2.91 mm²/s), melon seed oil (3.83 mm²/s) and tobacco seed oil (3.5 mm²/s) [ 11, 22, 37]. From Table 4, it can be observed that the KV value obtained in this paper fits well into the ranges of values specified by both ASTM D 6751 and EN 141214 standards. In addition, this value is 0.78 mm²/s higher and 0.73 mm²/s lower than that of PKO methyl ester (PKOME) and GNO methyl ester (GNOME), respectively. Therefore, it is evident that the KV (3.69 mm²/s) of HOME results from the binary mixture of PKO and GNO. To this effect, an improvement is witnessed for the KV of GNOME from 4.42 mm²/s to 3.69 mm²/s for HOME, though higher values such as 4.6 mm²/s [ 31] and 5.908 mm²/s [ 30] were reported for GNOME. The transesterification of the mixed oil (HO) has brought about a reduction of 88.46% for its KV.

Density

Fuel injection equipment operates on a volume metering system, hence, a higher density of biodiesel results in the delivery of a slightly greater mass of fuel. The density of HOME is 865.7 kg/m³ (Table 4) which satisfies the EN 14214 specification as there is none for ASTM D6751 standard. As given in Table 4 for comparison, the density of HOME represents a 9.3 kg/m³decrease in the density of PKOME and a 17.2 kg/m³ increase in the density of GNOME. It can also be deduced that the density of HOME (865.7 kg/m³) represents a reduction of the density of the starting oil by 4.94% via the transesterification process.

Flash point

The flash point (FP) of a fuel is the temperature at which it ignites when exposed to a flame or spark. Biodiesel offers safety benefits over petroleum diesel because it is much less combustible with a higher FP compared to petroleum diesel. The FP of HOME is 170.2°C which is 93.2°C (121.04%) higher than the value of FP for conventional diesel (77°C). The value of FP obtained in this paper is significantly higher than the minimum specifications recommended by both the ASTM D6751 and EN 14124 standards as presented in Table 4. The FPs of PKOME and GNOME are slightly below that of HOME as observed in Table 4. This is probably caused by the improved FA composition of the feedstock (HO) that enters the transesterification process.

Higher heating value

Higher heating value (HHV) is the energy content of the fuel. This determines the suitability of biodiesel as an alternative to diesel fuel. As provided in Table 4, HOME has a HHV of 40.2 MJ/kg. This result is 0.1 MJ/kg higher and 0.36 MJ/kg lower than the values of HHV presented in Table 4 for GNOME and PKOME, respectively. For the ASTM D6751 and EN 14124 standards, specifications about HHV of biodiesel do not exist, therefore, it can only be compared with that of diesel fuel (44 MJ/kg) which is slightly higher.

Cetane number

Cetane number (CN) is a dimensionless parameter used for the determination of diesel fuel quality, especially the ignition quality. It measures how easily ignition occurs and the smoothness of combustion. A high CN implies short ignition delay. The CN of HOME is empirically determined to be>60. As given in Table 4, the CN of HOME fulfills the minimum specifications recommended by the ASTM D6751 and EN 14124 standards. The CN of biodiesel depends on the structure of the fatty acid methyl ester (FAME) components [ 27]. The longer the FA carbon chains and the more saturated the molecules, the higher the CN [ 34]. The high CN obtained in this paper can be attributed to the presence of oleic methyl ester (47.80%; CN=59), lauric methyl ester (24.23%; CN=67) [ 27] amongst other methyl esters having a CN of more than 60. As provided in Table 4, the CNs of PKOME (58) and GNOME (53.59) are relatively high which are traceable to their FAME compositions (Table 1) and equally responsible for the high CN of HOME.

Acid value

Acid value (AV) is a measure of the FFA content in the biodiesel. The AV of HOME is 0.141 mg of KOH/g (Table 4). This result is appreciably below the maximum limit of 0.5 mg of KOH/g prescribed by both the ASTM D6751 and EN 14124 specifications. Obviously, the AV of the starting oil used in this paper was reduced by 90.14% through the transesterification process. A high AV makes the fuel prone to polymerization and acts as catalyst for hydrolysis which can eventually damage the engine.

Iodine value

Iodine value (IV) is an important biodiesel parameter that determines the unsaturation degree of the fuel. This parameter greatly influences fuel oxidation and the type of ageing products and deposits formed in diesel engine injectors. HOME is measured to have an IV of 0.457 g I₂/g. This result satisfies the maximum specification of 1.20 g I₂/g specified by the EN 14214 standards as there is none stipulated by ASTM D6751, as shown in Table 4. The relatively low IV of HOME results from the considerably low content of polysaturated FA (15.39%) present in the HO.

Cold flow properties

Two key parameters for low temperature application of a fuel are cloud point (CP) and pour point (PP). CP is the temperature at which wax first becomes visible when the fuel is cooled. PP is the lowest temperature at which the fuel can still be moved. As PKO contains more saturated FAs and GNO comprises more unsaturated FAs, their mixture may possibly produce a biodiesel with better fuel quality. As can be observed in Table 4, the CP and PP of HOME are determined to be 3.1°C and –1°C, respectively. These values are within the values obtained for the biodiesels produced from the parent oils mixed to synthesize HOME (Table 4), which may be accredited to the FA composition of the binary mixture. There is no limit described in ASTM D 6751 and EN 14214 for CP of biodiesel with regard to specification, but its value has to be reported. Nigeria is in the tropics, a CP of 3.1°C and a PP of –1°C seem to be safe for the application of HOME as fuel throughout the year. The presence of a moderately high content of unsaturated FAMEs in the HO biodiesel (Table 1) causes the slightly high CP and PP (Table 4).

Oxidative stability

Oxidative stability (OS) is a very significant fuel parameter because it provides relevant information about storage. The rate of oxidation depends on the number and position of double bonds, since alkylic hydrogens are more susceptible to oxidation. In this paper, the OS of HOME is determined to be 5.1 h. This result shows that HOME fulfills the minimum requirement of 3 h stipulated by ASTM D6751 while the 6 h specified by EN 14214 is not met. As seen in Table 4, only PKOME is reported to have satisfied the specifications set by both standards, which could have really influenced the OS of HOME prepared in this paper. As presented in Table 1, a relatively large amount of saturated FAMEs and a fairly high content of polyunsaturated FAMEs contributed to the OS value obtained for HOME in this paper. Thus, the binary mixture of oils used for biodiesel production in this paper has reasonably improved the OS of GNOME and the addition of antioxidants to HOME will improve the OS if the EN 14214 specification is to be satisfied.

Conclusions

This present paper explored the prospect of using hybrid feedstock for biodiesel production in the tropics due to the availability of feedstock and possible production cost reduction. Biodiesel was synthesized from the binary mixture of PKO and GNO at 50:50 (v/v) with methanol using NaOH as catalystunder the classical reaction conditions. The HO obtained mainly consists of oleic (35.62%) and lauric acids (24.23%) with 47.80% saturated FA and 52.26% unsaturated FA contents. An average methyl ester yield of 86.56% was obtained. The fuel properties of HOME determined were found to meet both the ASTM D6751 and EN 14214 specifications. The OS, CN and KV of HOME were improved compared with those of GNOME biodiesel, which could be ascribed to the improved FA composition of the HO. The comparatively low KV (3.69 mm²/s) of HOME is worth mentioning compared with those reported for most biodiesels (≥4.0 mm²/s). Besides, the KV of HOME represents an 88.46% of reduction of the KV of the binary mixture of the starting oils.

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